Several publications are cited in this application by numerals in parentheses in order to more fully describe the state of the art to which this invention pertains. Certain patents are also cited. The disclosure of each of these citations is incorporated by reference herein.
A basic function of innate immunity is restriction of the early proliferation of infectious agents (1, 2). Numerous molecules and effector cells conspire to restrict this initial spread of an infectious focus. Some examples of first line host defense molecules include antimicrobial peptides, natural antibodies, complement proteins, lipopolysaccharide binding protein (LBP), soluble receptors and collectins (3-5). The collectins are multimeric carbohydrate recognition domain-containing molecules with collagen stalks that include the pulmonary surfactant proteins-A and -D, conglutinin, CL-43, CL-46, and the MBL (6-9). MBL appears to be a prototypic pattern recognition molecule that is able to recognize the molecular patterns that decorate a wide range of microorganisms.
Infectious agents that are recognized by MBL include certain Gram positive and Gram negative bacteria, yeast, parasites, mycobacteria, and viruses (7, 10).
MBL was first surmised to play a role in host defense based on its overall structural similarity with the first complement component C1q (11, 12). Next, in vitro observations demonstrated that MBL could bind and opsonize bacteria as well as the yeast cell wall product mannan (13). The idea that a relative lack of MBL might predispose the host to infection was based on the description of an MBL-dependent opsonic defect in human serum that correlated with a phenotype of recurrent infection (14). These patients were found to have one of three amino acid substitution single nucleotide polymorphisms (SNPs) in exon 1 of the MBL gene that disrupt the collagen helix (15). It appears that the disordered collagen chain acts like a dominant negative resulting in a decrease in circulating levels of MBL that can activate complement. More detailed analysis of the MBL gene has revealed seven distinct MBL haplotypes in humans, four of which (LYPB, LYQC, HYPD, and LXPA) dictate low serum levels (16). Interestingly, there is a high rate of haplotype variation in various human populations with a range of heterozygosity from 15% in Caucasians to 30% in certain African populations (17, 18).
Importantly, MBL seems to be able to distinguish species self as well as altered self, e.g. in the form of apoptotic cells from non-self (19). The specificity that allows the distinction of surfaces of virally infected cells and transformed cells from normal host cells depends on both fine recognition of molecular patterns and a macropattern (3). The macropattern appears to be dictated by the spatial orientation of the carbohydrate binding domains and the differences in geometry of the sugars that adorn microorganisms versus host glycoproteins (3, 20, 21). MBL is able to activate complement via a novel mechanism that co-opts the mannose-binding lectin associated serine protease (MASP) (22, 23), MASP-2 which then mimics the classical pathway convertase to cleave the third complement component (C3) (23). In this way the MBL complement pathway is activated in an antibody-independent manner. MBL therefore has many functional properties that are reminiscent of an antibody and in fact MBL is considered as an opsonin (24-26).
The initial response to infection is a complex interaction between a variety of pattern recognition molecules that trigger the downstream physiological cascades of clotting, cytokine, and chemokine release and interface with effector cells such as neutrophils (27, 28). Neutrophils express complement receptors, MBL receptors (collectin receptors) (29, 30) and the receptor for LBP (31).
Wright and colleagues linked humoral and cellular interactions and drew attention to the importance of co-operative interactions between neutrophils and opsonins in combating infection (32, 33). More recent examples that have exploited the use of null animals to explore such interactions and are germane to this present study, include the interaction of LBP and neutrophils in resistance to intraperitoneal Salmonella infection (34, 35). A similar synergistic interaction between neutrophils and MBL is suggested by clinical observations that chemotherapy-induced neutropenic patients with haplotypes that specify low serum MBL levels (9, 36, 37) appear more susceptible to infection (38). These clinical observations together with in vitro studies suggest that MBL plays a key role as an ante-antibody in first line host defense (39, 40) and support a role for MBL in combating infection in vivo.